1. Field of the Invention
The present invention relates to a press-fit terminal used for electronic equipment and a press-fit joining wiring board into which the press-fit terminal is press-fitted. More particularly, the present invention relates to a press-fit terminal and a press-fit joining wiring board characterized in that an influence on the wiring board, due to press-fitting, can be suppressed in the case where the press-fit terminal is press-fitted into a through-hole formed on the wiring board, which is arranged in an electronic control unit used in a high temperature environment, and the press-fit terminal is electrically joined to the through-hole.
2. Description of the Related Art
Conventionally, there are provided electric control units (ECU) for controlling devices such as an engine and the like which are mounted on a vehicle. Concerning on ECU, various control functions for controlling the devices are collected into one unit, and a plurality of ECU are mounted on the vehicle. Each ECU includes: a control circuit portion including a microcomputer which operates according to electronic information detected by each sensor, the control circuit portion conducting logical control; and a power circuit unit for controlling electric power supplied to an actuator which drives an object to be controlled according to the result of the calculation. The above ECU, to be mounted on a vehicle, is disclosed, for example, in the official gazette of JP-A-2000-323848.
In this conventional ECU to be mounted on a vehicle, the control circuit unit is formed into a control board, and the power circuit unit is built in a module. In the process of assembling ECU to be mounted on a vehicle, in the case of incorporating the control board into ECU, connection terminals, which are vertically arranged on the module in which the power circuit is built, are inserted into through-holes formed on the control board. After that, the connection terminals are soldered to conductive members formed in the through-holes by means of soldering. By this method, the connection terminals are electrically connected to a wiring pattern formed on the control board. At the same time, the control board is fixed in a resin case. However, as the above constitution is adopted, the assembling work is complicated.
Therefore, in order to positively assemble the control board and the connection terminal, and at the same time, in order to reduce the amount of time necessary for assembling and, further, in order to prevent the working environment from deteriorating, a board connection is proposed in which press-fit terminals are used for the connection terminals. A case in which the press-fit terminals are used for the board connection of an ECU is disclosed, for example, in the official gazette of JP-A-10-208798.
In order to conduct the board connection easily, a plurality of through-holes are formed at predetermined intervals on the control board incorporated into ECU. An inner circumferential face of each through-hole is plated with conductive material. On the other hand, the press-fit terminals are vertically arranged at predetermined intervals corresponding to the through-holes while one end part of each press-fit terminal is being embedded in the housing. For example, the needle-eye type press-fit terminals are used for these press-fit terminals. These press-fit terminals are press-fitted into the through-holes provided on the control board and connected to the board by means of press-fit joining.
Other than the press-fit terminals described above, for example, as disclosed in the official gazette of JP-B-6-28174 and others, press-fit terminals are proposed, the quantities of elastic deformation of which are uniform when the press-fit terminals are press-fitted into the through-holes. But, when these press-fit terminals are press-fitted into the through-hole, a portion of high rigidity formed in an upper portion of the press-fit terminal is located outside the through-hole.
On the other hand, concerning the control board mounted on the above electronic control unit, there is a demand for using a material, the coefficient of thermal expansion of which is low, from the viewpoint of enhancing the reliability of connection. In order to meet the demand, ceramic boards, ceramic-resin compound boards, fiber compound boards and the like have been developed. However, no boards exist which satisfy both the demands of reducing the coefficient of thermal expansion and enhancing the machining property. Therefore, laminated plates covered with metallic foil, in which a prepreg layer obtained when epoxy resin is impregnated in a sheet-like base material and dried and a sheet of metallic foil put on the surface are integrated into one body being formed with heat and pressure, are used as boards.
Therefore, for example, as disclosed in the official gazette of JP-A-8-309920, laminated boards have been proposed in which the elastic modulus of a resin is lowered by adding a flexible material to epoxy resin, so as to lower the elastic modulus of the resin, and thermal expansion in the surface direction of the laminated plate is suppressed. In the case of this laminated board, in order to prevent the metallic foil from peeling off from the laminated board by the thermal expansion of the board, fine particles having rubber-elasticity are contained in the resin, so that stress generated in the resin by thermal expansion can be absorbed and reduced. Due to the foregoing, thermal expansion in the plane direction of the laminated board can be reduced.
In this connection, in the case of press-fitting the press-fit terminals into the through-holes provided on the control board, minute damage may be caused in an opening portion of each through-hole formed on the control board by stress concentration in the case of press-fitting the press-fit terminal. Usually, the control board is composed of a laminated board made in such a manner that a large number of sheets in which glass fibers are combined with each other in the longitudinal and the lateral direction and impregnated in epoxy resin and compressed and solidified. Therefore, when the press-fit terminals are press-fitted, the opening portion of the through-hole is given a strong force in the face direction of the board. Accordingly, in the periphery of the opening portion of the through-hole, the laminated sheets are peeled off, that is, the base material is damaged.
In the case of an electronic control unit arranged in an engine compartment of an automobile which is operated in a severe operation environment, for example, in an environment in which the surrounding temperature is high and the humidity is also high, moisture tends to be absorbed in the damaged portion on the control board. As a result, copper ions are dissolved out from the conductive material, and the deterioration of insulation of the board proceeds. Recently, as various control functions are required for the electronic control unit, the density of the control board has increased and, further, the size of the control board has been reduced, the number of electric connecting portions on the control board has increased and the distance between the through-holes has decreased. Accordingly, the deterioration of insulation of the board is facilitated, which causes a serious problem.
Especially, in the case of the control board of the electronic control unit arranged in an engine compartment in which intense vibration occurs, in order to endure the intense vibration, it is necessary to increase the holding force of the press-fit terminals. However, when a press-fitting margin is increased so as to enhance the holding force of the press-fit terminals, a heavy load is given to the board in the case of press-fitting, which results in damage. On the contrary, in order to decrease a load given to the board in the case of press-fitting, it is possible to reduce the press-fitting margin. However, this method is disadvantageous in that the holding force of the press-fit terminals is decreased.
Therefore, it is an object of the present invention to provide a press-fit terminal characterized in that: the press-fit terminal can be applied to a commonly used control board; it is possible to reduce a load given to an opening portion of the through-hole of the control board in the case of press-fitting; a high intensity of the holding force can be maintained after the completion of press-fitting; and the reliability of the press-fit terminal is higher than that of a conventional press-fit terminal.
It is another object of the present invention to provide a press-fit joining wiring board capable of absorbing stress given from the outside so as to reduce a load generated on the board in the case where the press-fit terminal is press-fitted into a through-hole, which is arranged on a wiring board in an electronic control unit used in an environment of high temperature, so that the press-fit terminal can be electrically connected to the through-hole. It is still another object of the present invention to provide a board structure in which a conventional press-fit terminal can be used as it is and stress given to the board from the terminal can be absorbed by the board.